I've got a Telstra landline purely for ADSL. I've had it at this address for over 8 years and never plugged a handset into it (in fact I don't even own a handset).
I noticed on a recent bill that I'd been charged for some calls - calls to 13 numbers, directory assistance, the weather - as well as a number of calls to mobiles and landlines at no charge.So I checked back over the past year of bills and from April 2019 these random call charges started appearing. (I never noticed these charges on my bill because the kids sometimes order Telstra bigpond rentals etc.).
The kids haven't plugged anything in, apart from not owning a handset, they wouldn't even know how LOL
I contacted Telstra who can't explain how I can be billed for calls when I don't have a handset plugged in. I've been told I need to call Tech support from the line so they can run diagnostics. Bit hard when I don't own a handset. I also asked them to please run a query from their billing system to check exactly what calls I've been charged for (so I don't have to manually go back over 8 years of bills) and they promised they would, but 2 weeks later I've heard nothing.